I tend to use chronic daily headache and migraine interchangeably because my chronic migraines are thought to have transformed into daily headaches. I’m trying to stop doing this, though, because migraine is not the only instigator of CDH. In fact, the International Headache Society has identified 24 different causes for it.
American Family Physician published an article in December 2004 that describes these causes in the US and Europe. In 53% of cases, CDH begins from chronic tension-type headaches. Another 31% are chronic migraines that have transformed into daily headaches. In both type, the headaches gradually change from being distinct events to constant. Another common cause with a gradual onset is medication overuse, a.k.a. rebound headaches.
For the majority of those with CDH, one or more of the above classifications will fit. But for others, CDH may be brought on suddenly by head or neck trauma, flu-like illness, surgery, meningitis, or some other medical illness. I don’t know much about this second set of causes and I don’t really know how they are treated. But I expect that we experience very similar things with the different types of CDH.